This application is based on and claims the benefit of German Patent Application No. 199 60 205.0 filed Dec. 14, 1999, which is incorporated by reference herein.
The present invention relates to a device for signal transmission between two terminals having at least one cable, positioned between them and accommodated in an essentially circular cartridge, from which further cables can be attached to the two terminals and whose length is greater than the distance of the two terminals from one another, in which device at least one of the two terminals is movable relative to the other and in which the cartridge consists of a rotor rotatable around its axis and a fixed stator which delimit an acceptor space serving to accept the cable, as described in published European patent application no. EP 0 417 350 A1.
This type of device is, for example, used for the transmission of a signal to activate the airbag of an impact protection system for motor vehicles, but is also used for sliding door cabling or steering mechanism cabling of motor vehicles. For this purpose the device is, as a rule, housed in the steering wheel of a motor vehicle for transmission of an electrical or optical signal. xe2x80x9cCablexe2x80x9d in the sense of the invention can thus be an electrical or optical cable. An essential problem for this device is signal transmission between fixed and movable parts of the motor vehicle. The sliding contacts and/or slip rings serving for current transmission, which have been long known for such cases, are subject to abrasion and are particularly disadvantageous at low current intensities due to the fluctuating transmission resistances.
In the known device according to the above-mentioned European patent application EP 0 417 350 A1, electrical current is used for signal transmission. The current transmission occurs through, e.g., a flat ribbon cable wound into a wound package like a spring barrelxe2x80x94referred to as xe2x80x9cFRCxe2x80x9d hereinafter. During relative rotational movement of the two terminals connected by the FRC, the wound FRC xe2x80x9cbreathesxe2x80x9d like the spring of a clock. The windings of the wound FRC are compressed to a smaller diameter in one rotational direction. In the other rotational direction, they open back up to a larger diameter. The windings of the FRC thereby rub on the bordering surfaces of the stator and rotor.
In the FRC accommodated in the cartridge of such devices, two or more conductors are positioned at a distance from one another. The width of the FRC thus increases as the number of conductors increases if the electrical properties of the FRC, particularly the ohmic resistance, are to remain unchanged for a constant length of FRC. Because the cable is inserted in the acceptor space of the cartridge edgewise, the word xe2x80x9cheightxe2x80x9d will be used instead of the word xe2x80x9cwidth,xe2x80x9d meaning the axial height of the mounted cable in the acceptor space of the cartridge. The dimensions of the cartridge according to the previously mentioned EP 0 417 350 A1 are selected for the use of a cable with a fixed number of conductors. This is also true for all other known cartridges. If a different number of conductors is necessary than is present in the cable provided, either the dimensions of the cartridge must be adapted to a correspondingly shorter or taller cable, or only a part of the conductors present in a suitable cable will be used. Both variants are costly.
It is an object of the invention to provide a device of the type initially described which is usable, within wide limits, for FRC""s with any desired number of conductors.
This and other objects of the invention are achieved by a signal transmission device wherein:
a compensation body with variable axial height is applied to at least one of the surfaces of the rotor and stator bordering on the acceptor space and
for a constant axial height of the acceptor space of the cartridge and a variable axial height of the FRC, the axial height of the compensation body is dimensioned in such a way that the free space remaining between the FRC and the rotor and/or stator of the cartridge in the axial direction is essentially filled by the compensation body.
With this device, a cartridge with constant dimensionsxe2x80x94particularly those of the acceptor space for the FRCxe2x80x94can be used for many different requirements. The number of conductors of the particular FRC to be used can thus be advantageously adapted to the particular application. Therefore, FRC""s of varying heights can be used. The axial free space within the acceptor space of the cartridge, which is therefore variable, is filled out for any application by a compensation body of a matching height. The sum of the axial height of the FRC and compensation body remains essentially constant in all cases. The FRC therefore cannot move in the axial direction by enough to stress the contact points, even if the height of the FRC is small in relation to the height of the acceptor space. Tangling of individual windings of the FRC is also thus prevented. The specific matched compensation body can also always assume a muffling function in addition to its function as a height compensator if suitable materials are used.